Does the type of encryption you use on your network affect the speed of your connection?

WPA has more overhead than WEP. It employs a 128-bit rotating key vs. 40 bits, adding more bits to each packet. That reduces payload bandwidth somewhat, so you’re not seeing things. The best bet for performance is to have an unsecured network, which you probably don’t want to have if you live within 300 feet of anyone else or a roadway.

If it’s a concern for you, you might upgrade to 802.11n, which has higher throughput overall.